When I split to mono and invert one of the mono tracks, they simply cancel out and create silence. 
I'm wondering if it because the track, to start with, isn't true stereo, and converting to mono basically does nothing? Even if I rip "stereo" from the CD, if it was recorded mono then that kinda screws things up.
You can verify if the track is mono by viewing it on a phase display ( a straight line would indicate mono. ).
How are you splitting to mono.? ( I haven't looked at the link yet ). If the tracks are totally cancelling, then they are exactly the same.
Vocal removal (kinda) works by mixing the the common vocal information back with the original information except out of phase. Since vocals are usually in the center ( common to both stereo tracks ), if you mix that back in but out of phase, the center information cancels. The music tracks tend to be spread across the stereo field so will not be the same in both tracks hence it doesn't cancel (as much). If vocals have reverb. added to them, the reverb. is usually spread across the channels as well so that will be left behind too.
Is there not a preset for vocal removal in Audacity you can try?
A channel mixer configuration something like this:
New Left channel = 100 percent Left + negative 100 percent Right
New Right channel = negative 100 percent Left + 100 percent Right Inverting the input (right) channel
These settings will collapse the sound field down to mono.
Where for example a Left value of 50 and a Right value of 50 results in a new channel that contains equal audio from both the current L and R channels.
edit: After looking at the video, the "first step" is doing what the channel mixer config. above does. If your original "rip" is two channels, zoom in and look at the waveforms. If they look identical, they are more than likely mono. ( By the way in the video BP stands for Band Pass. )
(There were two posts while I was typing my original post)
